Hindered rotation, about single bonds

Atropisomers - Atropisomers are stereoisomers resulting from hindered rotation about single bonds where the steric barrier to rotation is high enough to allow for the isolation of the conformers <2004T4335>. For examples see Section 2.3.43.2. 

DNMR also reveals hindered rotations about single bonds in addition to rotation about the M=C bond in some conventional carbene complexes, such as 26-28. Restricted rotation around the C-C bond between the / 3t>-carbon and the a-carbon in both Fe=CHAr and Ru=CHArwas observed with AG 40-50kJ mol . " " ... 

Atroplsomers Stereoisomers that result from sterically hindered rotation about single bonds when energetic barrier for rotation is high enough that conformers may be isolated. 

There is no plane of symmetry and the molecule is chiral many such compounds have been resolved. Note that groups in the para position cannot cause lack of symmetry. Isomers that can be separated only because rotation about single bonds is prevented or greatly slowed are called atropisomers. 9,9 -Bianthryls also show hindered rotation and exhibit atropisomers. °... 

These are rotations about single bonds. The calculation is carried out first for the rotation which is assumed to be free (subscript f), then the fact that the rotation is not free, but hindered (subscript h) by the other atoms of the molecule is taken into account. 

One important aspect which is not necessarily apparent from this use of the central limit theorem is that in the general case of hindered rotations, or the interdependence of rotations about single bonds, the step lengths / are functions of temperature. 

High-performance liquid chromatography can sometimes separate atropi-somers, i.e., a compound with highly hindered rotation about a single bond, producing effectively two molecular species. 

Normally, rotation around single bonds has a barrier below 5 kcal mol and occurs faster than the NMR time scale. Rotation around the double bond of alkenes, on the other hand, has a barrier that is normally above 50 kcal mol and is slow on the NMR time scale. There are numerous examples of intermediate bond orders, whose rotation occurs within the NMR time scale. Hindered rotation about the C —N bond in amides such as A, A-dimethylformamide (5-4) provides a classic example of site exchange. At room... 

It is possible to have a chiral centre about a single bond so long as there is restricted rotation. An example of this hindered rotation occurs in biphenyl derivatives, which have the same extended tetrahedral geometry as the spiro and allene compounds above. Isomers that may be resolved only because of hindered rotation about a single bond are called atropisomers. 

A further example of hindered rotation about a single bond occurs in conjugated dienes. The simple case of buta-1,3-diene exists in two forms, which are described as transoid and cisoid. The transoid form is the thermodynamically more stable form as there is less steric interaction between the terminal methyl groups. 

E. Hindered rotation about formal single bonds... 

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